1. Field of the Invention
Embodiments of the present invention generally relate to an apparatus and method for intervening in offshore pipelines. Embodiments also relate to an apparatus and method for drilling and casing an offshore wellbore.
2. Description of the Related Art
Hydrocarbon production occurs either directly from the earth or from the earth below a body of water. Production directly from the earth is typically termed a “land production operation,” while production from the earth below a body of water is ordinarily typically termed an “offshore production operation” or a “subsea” production operation.” To obtain hydrocarbons in either a land production operation or an offshore production operation, casing is inserted into a drilled-out wellbore within the earth formation. Casing isolates the wellbore from the formation, preventing unwanted fluids such as water from flowing from the formation into the wellbore. The casing is perforated at an area of interest within the formation which contains the desired hydrocarbons, and the hydrocarbons flow from the area of interest to the surface of the earth formation to result in the production of the hydrocarbons. Typically, hydrocarbons flow to the surface of the formation through production tubing inserted into the cased wellbore.
Casing is inserted into the formation to form a cased wellbore by a well completion operation. In conventional well completion operations, the wellbore is formed to access hydrocarbon-bearing formations by the use of drilling. Drilling is accomplished by utilizing a cutting structure that is mounted on the end of a drill support member, commonly known as a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and its cutting structure are removed from the wellbore and a section of casing is lowered into the wellbore. An annular area is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. A cementing operation is then conducted in order to fill the annular area with cement. Using apparatus known in the art, the casing string is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with a cutting structure on a drill string. The drill string is removed. A first string of casing or conductor pipe is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed, or “hung” off of the existing casing by the use of slips which utilize slip members and cones to wedgingly fix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
As an alternative to the conventional method, drilling with casing is a method often used to place casing strings of decreasing diameter within the wellbore. This method involves attaching a cutting structure in the form of a drill bit to the same string of casing which will line the wellbore. Rather than running a cutting structure on a drill string, the cutting structure or drill shoe is run in at the end of the casing that will remain in the wellbore and be cemented therein. Drilling with casing is often the preferred method of well completion because only one run-in of the working string into the wellbore is necessary to form and line the wellbore per section of casing placed within the wellbore.
Drilling with casing is especially useful in drilling and lining a subsea wellbore in a deepwater well completion operation. When forming a subsea wellbore, the initial length of wellbore that has been drilled is subject to potential collapse due to soft formations present at the ocean floor. Additionally, sections of wellbore that intersect areas of high pressure can cause damage to the wellbore during the time lapse between the formation of the wellbore and the lining of the wellbore. Drilling with casing minimizes the time between the drilling of the wellbore and the lining of the wellbore, thus alleviating the above problems.
After production of the hydrocarbons at the surface of the earth formation, the hydrocarbons must be stored at a location and subsequently processed to remove undesired contaminants in the hydrocarbons and to produce the desired product. In land production operations, one option for storage and processing involves storing the hydrocarbons at a tank beside the wellbore, removing the hydrocarbons at time intervals from the storage unit with a mobile storage unit, physically transporting the hydrocarbons with the mobile storage unit to a processing unit, removing the hydrocarbons from the mobile storage unit and into the processing unit, and then processing the hydrocarbons at the processing unit.
Another option for storing and processing the hydrocarbons in land production operations includes using a pipeline connected to the production tubing. The hydrocarbons flow from the formation through the perforations, into the production tubing, through the pipeline, and into a storage and processing unit at a remote location. The storage and processing unit typically receives multiple pipelines from multiple land production operations at various wellbores to allow storage and processing of hydrocarbons from multiple locations at one facility without the need for physically transporting the hydrocarbons to the processing unit.
The above options for storage and processing of hydrocarbons produced during land production operations are feasible because of the unlimited space available for storage units and processing units on the land. Offshore production operations, however, require alternative storage and processing methods because of the limited space allotted to hydrocarbon production at the surface of a body of water. Thus, methods for storing large quantities of hydrocarbons at a remote location are currently practiced.
Offshore wells are often drilled and completed by use of a drilling rig. The drilling rig includes legs which rest on the floor of the body of water and support a rig floor. A hole is located in the rig floor of the drilling rig through which supplies for completing and drilling the wellbore, such as a drill string and casing strings, may be inserted and lowered into the body of water. The wellbore is typically drilled out by use of the drill string, then strings of casing are placed within the drilled-out wellbore to form a cased wellbore. Perforations are created in the casing and the formation as described above. A riser, which is piping which spans the distance of the water from the ocean floor to the surface of the water, is ultimately inserted at the top of the cased wellbore. Because drilling rigs are relatively expensive to maintain above the wellbore after the completion operation, the drilling rig is removed from its location above the completed wellbore and employed to drill a subsequent wellbore at a different location. At this point, production of the hydrocarbons and subsequent storage of the hydrocarbons becomes an issue.
One method for producing and storing hydrocarbons in offshore operations involves first building a production platform on the ocean floor. Like the drilling rig, the production platform includes a platform supported on legs which extend to the ocean floor. Production tubing is lowered from a hole which exists in the production platform into the riser and the cased wellbore to the area of interest which contains the perforations, then the hydrocarbons flow through the production tubing to a storage unit located on the production platform. The production platform is usually not large enough to accommodate the large volume of hydrocarbons which flow through the production tubing to the production platform; therefore, the production platform must only store hydrocarbons until a tanker arrives to transport the hydrocarbons from the storage unit to a larger storage and processing unit at another location. This method is expensive because each production platform above each wellbore which must be constructed and maintained represents a relatively large expense.
Alternatively, a subsea well intervention vessel having processing equipment coupled to storage tanks and having well intervention equipment may be utilized to produce hydrocarbons through coiled tubing drilling or to store or process hydrocarbon mixtures produced from underbalanced drilling, as described in U.S. Publication Number 2003/0000740 published on Jan. 2, 2003, filed by Haynes et al. and entitled “Subsea Well Intervention Vessel”, which is herein incorporated by reference in its entirety. The intervention equipment is theoretically capable of reentering existing production wells without changing the wellbore from its production mode.
As a more economic alternative to installing a production platform above each wellbore, a second method of producing and storing hydrocarbons in an offshore operation is more often practiced. Rather than building and maintaining a production platform for each wellbore, the median step in the production and storage operation which includes the production platform is eliminated by satelliting. Satelliting involves installing pipelines at each wellbore and routing all of the pipelines to a common storage and processing location, typically termed a “satellite unit,” through the pipelines. The pipelines remain underwater from the wellbore until reaching the storage and processing unit.
Special problems are currently encountered when satelliting. The hydrocarbons must often travel long distances through the underwater pipelines to reach the satellite unit. The water through which the hydrocarbons must pass is very low in temperature, especially at the ocean floor where the pipelines are commonly placed. Because of the cold temperatures within the water, flowing the liquid hydrocarbons underwater for long distances is often challenging. One problem which may result from the cold temperature of the water involves the viscosity of the hydrocarbons. Viscosity of liquid hydrocarbons increases as the hydrocarbons decrease in temperature. The higher the viscosity of the hydrocarbon liquid, the lower the flow rate of the hydrocarbon liquid becomes. Therefore, the colder the water surrounding the pipelines becomes, the more difficult or impossible flowing the hydrocarbons from the wellbore to the storage unit becomes. Decreasing viscosity of the liquid hydrocarbons flowing in the pipeline may ultimately cause blockage within the pipeline, reducing or halting hydrocarbon production.
A second problem which may result from the cold temperature of the water involves the changing temperatures of the hydrocarbons during their production. Within the wellbore, temperatures are high, causing the hydrocarbons to possess a high temperature. The hydrocarbons within the wellbore may contain both the liquid and gas phases. As described above, the environment within the water when the hydrocarbons travel through the pipelines consists of low temperatures. Then, when the hydrocarbons are flowed from the pipeline up to the satellite unit, the temperature of the environment of the hydrocarbons becomes increasingly higher as the temperature increases with decreasing depth within the water. These temperature variations when using pipeline to transport produced hydrocarbons to the satellite unit often result in precipitation of the hydrocarbons on the inside of the pipeline. Eventually, the precipitation build-up may result in partial or total blockage of the flow path through the pipeline, decreasing or stopping hydrocarbon production. Reduced hydrocarbon production decreases the profitability of the wellbore.
Other problems which require pipeline intervention to reduce blockage include paraffin deposits which often build up in the pipelines due to the presence and flow of oil, as well as gas hydration when gas is present in the hydrocarbon stream. Some form of pipeline intervention must occur to reduce blockage within the pipeline caused by cold and variable temperatures. Pipeline intervention may also be necessary to repair holes or tears in the pipeline caused by corrosion of the pipeline or holes, tears, or bends due to physical assault of the pipeline.
Currently, an intervention operation requires a remotely operated vehicle to raise the pipeline off the ocean floor, cut out the damaged section of pipe, install pipe connections on the cut ends of the pipeline, and then install and connect a new section of pipeline. Other intervention operations require stoppage of the flow of hydrocarbons through the pipeline to introduce treatment fluid into the pipeline to remove blockage within the pipeline. For the intervention process, to prevent hydrocarbons from escaping from the pipeline to the water, hydrocarbon flow must be halted during the intervention operation.
Pipeline intervention operations are costly. Because interventions require physical invasion of the interior of the pipeline, hydrocarbon flow must be halted to conduct an intervention. Stopping the flow of the hydrocarbons reduces the profitability of the well, as the equipment and labor required to produce the hydrocarbons is still funded while no hydrocarbon production is occurring to offset these costs. It is therefore desirable to allow hydrocarbon production during pipeline intervention operations in offshore wells.
Light intervention vessels are available which make it possible to conduct operations such as well servicing, e.g. well logging and general maintenance. Such vessels, however, cannot be considered appropriate platforms for interventions requiring drilling or hydrocarbon production as they are not sufficiently stable for such operations and are too small to handle the volumes of material that result from drilling. As such, the vessels must be supplemented with support vessels to receive produced hydrocarbons. Furthermore, light intervention vessels require large capital investments as compared with the returns that can be generated, particularly as they are highly vulnerable to bad weather such that intervention costs are relatively high and utilization time is relatively low. Even more cost is required to employ an additional support vessel. Because of the above disadvantages, no attempts have been made to use continuous casing to drill and line a wellbore from floating units or to allow hydrocarbon production during intervention operations in offshore wells.
Furthermore, while drilling with sections of casing minimizes the time between the drilling of the wellbore and the lining of the wellbore, it remains desirable to further minimize the time between the drilling and lining of the wellbore to decrease production costs and further prevent collapse of the formation during the time lapse. It is also desirable to provide an alternative to the costly drilling platform or the option of two vessels (one having the drilling equipment and one having storage equipment) by allowing drilling with casing completion operations as well as production operations to be conducted concurrently from the same structure.